1. Field of the Invention
The present invention relates generally to the molding of pre-formed fiber-reinforced resin components into composite parts that have multiple fiber orientations and/or one or more internal chambers. More particularly, the present invention is directed to molding of such pre-forms under high pressure.
2. Description of Related Art
Composite materials typically include fibers and a resin matrix as the two principal components. Composite materials typically have a rather high strength to weight ratio. As a result, composite materials are being used in demanding environments, such as in the field of aerospace where the high strength and relatively light weight of composite parts are of particular importance.
High strength metals and metal alloys have been used in the past to form aircraft parts and structures that are subjected to high stress loads. An advantage of using metal is that, in addition to being extremely strong, metal can be precisely machined so that the geometry of mating parts can be closely matched and tight dimensional tolerances can be met. Holes can also be precisely machined into metal structures to accept high tolerance fasteners and positioning pins.
The turbofan engines that are commonly used to propel aircraft include a number of parts that are subjected to extreme stress and which also must meet precise dimensional tolerances. One example of such a part is the outlet guide vane (OGV). These vanes are airfoil-shaped parts that are located in the by-pass area of a turbofan engine. The outlet guide vanes redirect radial airflow from the forward fan assembly into axial airflow that goes to the turbofan compressor. Each engine can contain as many as 100 or more outlet guide vanes. In order to function properly in this demanding environment, outlet guide vanes have been traditionally made from solid metal or metal alloy. Some outlet guide vanes have been made by surrounding a lighter core material with high-strength metal. More recently, outlet guide vanes have included a metal frame on which composite skins have been mounted. For example, see published U.S. Patent Application No. 2010/0209235 A1. These metal-based outlet guide vanes all tend to be relatively heavy. Accordingly, it would be desirable to provide a lighter-weight outlet guide vane that is made completely from composite material and which has the same structural properties and machining characteristics as the metal-based outlet guide vanes.
Making an outlet guide vane from composite materials presents a number of significant challenges. In order to ensure that the finished part is free of voids or other defects, it is generally necessary to use molding techniques in which pressure is applied to the composite material during the molding process. However, it is very difficult to produce molded composite parts that have tolerances which are as tight as the tolerances that can be obtained by machining a molded metal part. It also is difficult to machine composite parts without creating structural damage. The fibers tend to be disrupted and delamination can occur which greatly reduces the strength of the part. Composite parts that contain unidirectional (UD) fibers are commonly used in many structural parts due to the unique structural and strength properties provided by a UD fiber orientation. The high directional strength provided by UD fibers makes this orientation particularly attractive for use in making outlet guide vanes. However, composite parts that contain UD fibers are particularly difficult to machine without adversely affecting the strength of the part.
Another significant challenge is that the central airfoil of the outlet guide vane and the mounting flanges or platforms that connect the airfoil to the engine have unique and different design requirements. For example, the airfoil needs to be structurally strong to withstand high airflow loads. The flanges must be strong enough to hold the airfoil in place and they must also meet strict dimensional tolerances to provide secure and precise mounting to the engine.
A composite material has been developed that can be machined accurately to strict dimensional tolerances. This composite material is composed of randomly oriented segments of unidirectional tape that have been impregnated with thermosetting resin. This type of quasi-isotropic fiber material has been used to make high pressure molds and a variety of aerospace components. The material is available from Hexcel Corporation (Dublin, Calif.) under the trade name HexMC®. Examples of the types of parts that have been made using HexMC® are described in U.S. Pat. Nos. 7,510,390; 7,960,674 and U.S. patent application Ser. No. 12/856,210.
There is a continuing need to develop new processes and procedures for making composite parts that can be used in place of outlet guide vanes and other high performance parts that have traditionally been made from metal.